1. Field of the Invention
The present invention relates to a reversible (i.e., bi-directional) fan, wherein the direction of air flow is reversed by changing the direction of rotation of the blades of the fan. In particular, the present invention relates to a bidirectional fan having blades that are asymmetric in shape and whose orientation is adjustable.
2. Description of the Related Art
Bi-directional fans used in supply-exhaust ventilation conventionally comprise an impeller having multiple blades that extend radially outward from a central hub to a coaxial, annual shroud. The annual shroud surrounds the impeller such that inner diameter of the annual shroud is greater than the outer diameter of the impeller, allowing the blades to rotate within the shroud with the peripheral edges of the blades in close proximity to the shroud. The impeller is rotated about a central axis by an electric motor which is mounted coaxially and supported by support members which extend radially inward from the annual shroud to the electric motor. The support members are spaced axially from the fan blades so as to avoid mechanical interference therewith. Such bi-directional fans can be mounted within an air duct system or used in a window unit or a built-in wall unit.
In order to move air efficiently in the forward direction (air flow exiting the side of the fan opposite the motor) and the reverse direction (air flow exiting the motor side of the fan), the blades of the conventional bi-directional fan typically have a symmetric, S-shaped profile (i.e., the transverse cross-section of the blade normal to the radial direction). Specifically, the leading portion of the blade lies on one side of a straight line connecting the leading and trailing edges of the blade, and is concave toward the straight line, providing a cupped blade surface capable of efficiently moving air in the forward direction when the blade is rotated in one direction (e.g., counter-clockwise) about the central axis. The trailing portion of the blade lies on the opposite side of the straight line and is concave toward the straight line, providing a cupped blade surface capable of efficiently moving air in the reverse direction when the blade is rotated in the opposite direction (clockwise).
The conventional symmetric fan blade is suitable in ventilation systems where the operational time of the bi-directional fan is roughly 50% in the forward direction and 50% in the reverse direction, since the symmetric fan blade provides approximately the same efficiency in the forward and reverse directions (ignoring air flow differences resulting from air flow blockage caused by the motor). However, in the case of a bi-directional fan having an operational time that is more than 50% in the forward (or reverse) direction, it is desirable that the fan have a greater energy efficiency in the forward direction than in the reverse direction to conserve energy. That is, the overall energy efficiency of the fan is greater with an increase in energy efficiency in the direction of greater use, even at the expense of a corresponding decrease in energy efficiency in the direction of lesser use. Accordingly, there is a need for a bi-directional fan which provides increased operational efficiency in one direction when more than 50% of the fan operation time is in that direction.
The air flow and energy efficiency requirements of a particular ventilation system may impose operational constraints that cannot be met by known bi-directional axial fans. For instance, the forward (e.g., supply) direction air flow and energy efficiency requirements may demand a blade structure that is unacceptably inefficient in the reverse (exhaust) direction. Accordingly, there is a need for a bi-directional fan capable of meeting air flow requirements in one direction without unduly degrading performance in the opposite direction.
One difficulty in designing the blades of a bi-directional ventilation fan for particular operating conditions is that operational requirements (e.g., the percentage of operational time used in the forward and reverse directions) may change over time. Assuming the blades of a ventilation fan have been optimized for particular operating conditions, the fan energy efficiency would be significantly reduced if different operating conditions were to be adopted. Further, it would be very expensive to design and manufacture customized fan blades based on the specific operational requirements of each and every bi-directional ventilation system. Accordingly, there is a need for an adjustable, bi-directional fan capable of providing a variety of different forward and reverse air flow energy efficiencies for a variety of different operating conditions.